Thermal testing apparatus



Sept. 28,1943. H. P. KUEHNI THERMAL TE S T ING APPARATUS 2 Sheets-Sheet l Original Filed Oct. 17, 1941 TIME Irfventor Hans P Kuehnl,

by Has Attoney Sept. 28, 1943. H. P. KUEHNI 2,330,59

THERMAL TESTING APPARATUS Original Filed Oct. 17, 1941 2 Sheets-Sheet 2 nan nus Inventor: Hans P. K uehn'l His Attorney.

Patented Sept. 28, 1943 THERMAL TESTING APPARATUS Hans P. Kuelmi, Schenectady, N. Y.,a|signor to General vElectric Company, a corporation of New York Original application October 17, 1941, Serial No.

Claims.

My invention relates to apparatus for testing by measurement of thermal conductivity and is a division of my application Serial No. 415,434, flied October 17, 1941, PatentNo. 2,323,715. My invention is useful for determining whether a spot weld is good or bad by a measurement which depends upon the thermal conductivity of the weld for example. The invention may be used to determine the thickness of a known material and may be used for other purposes where the nature of the test being made depends upon the thermal conductivity of a material or materials.

The features of my invention which are believed to be novel and patentable will be pointed out in the claims appended hereto. For a better understanding of my invention reference is made in the following description to the accompanying drawings in which Fig. 1 represents a form of temperature responsive apparatus as applied to detecting the nature of spot welds; Fig. 2 is a detail view oi a temperature sensitive resistance element which may be used in the apparatus of Fig. 1. Fig. 3 is a wiring diagram or an electric measuring circuit that may be used with the apparatus of Fig. 1. Fig. 4 illustrates time cooling curves explanatory of my invention. Figs. 5

Divided and this application May 1, 1943, Serial No. 485,354 g and 6 represent another form of detector that may be used. Fig. 'I is a wiring diagram 01' an automatic timing feature which may be used with the apparatus of Fig. 5; and Fig. 8 represents ditierent thicknesses of boiler scale which may be detected with the use of my invention.

Referring now to Fig. 1, I0 represents an elon- H such as Duralumin in which there is a cavity containing an electric heating resistor l2 connected to a suitable source of current I3 through a switch I4 for maintaining the mass of metal I l at a high temperature. The circuit preferably contains an adjustablethermostat switch [5 for maintaining the temperature substantially constant at a desired high value. Except for the nature of the upper end of the device shown in Fig; 1, it is generally similar to an electric solder-' ing iron. In place of a soldering iron head I have provided a head consisting of a metallic tube 9 made of Duralumin or other high melting point metal or alloy. This tube 9 is snugly fitted into a cross bore in the exposed end of the rod ll so as to be in good thermal contact therewith. Into the opposite ends of tube 9 is fitted similar metal electrode elements I6 and I1, each containing similar thermal responsive devices such as thermocouples or temperature responsive resistance elements l8 and IS. The upper extremity 01' heat storage rod ll contains a central opening 20 communicating with the interior oi. tube 9 and through which electrical connections 2| and 22 to the resistance elements l8 are carried. The other common connection to both resistance elements is formed through a wire 23 and the metallic structure of the apparatus.

The resistance elements l8 and I! may comprise small nickel cups 24 which are pressed into cavities in the metal contact buttons l6 and IT. The cups are filled with baked sodium silicate or a baked mixture of iron oxide and borax or some other suitable resistance material 25 with the connecting wire forming anelectrode in the center. The resistance elements specifically mentioned have a negative temperature coeillcient of resistance. and the resistance drops rapidly with increasing temperature and in- 1 creases rapidly when the temperature drops.

These resistance elements are connected in a measurement bridge circuit as shown in Fig. 3.

Resistance elements It and I9 form two arms of the bridge and impedance coils 2s and 21 form the'other arms. The bridge is supplied with an A.-C. voltage'source 28 and measuring instrument 29 preferably of the rectifier type is connected across the bridge to detect and measure the unbalance thereof.

The detector is shown with metame contact grees C. and under this condition with the resistance elements It and I 9 at the same temperature and resistance the bridge is balanced and the instrument 2! reads zero.

when the button It is brought against the metal plate 30 assumed to beat room temperature of, say, 25 degrees 0., there occurs a transfer of heat from button II to plate 30. Such heat flows into plate 30 and away from the point of contact. and it also flows primarily through weld material at 3| into plate 32, and resistance element [9 will drop in temperature as compared v to resistance element It and the bridge will be- Other things being equal the rate of cooling of resistance element I! will depend upon the nature of weld 3|. If it is a good weld, heat will flow through it readily. If it is a poor weld, the heat transfer therethrough will be less and there will be less unbalance of the bridge and a smaller instrument deflection in a given short period of time. The operation may be pictured as in Fig. 4.

In Fig. 4 the ordinates represent temperature of resistance I9 and the abscissa represents time. Let T represent the original temperature of resistance I9 before button I6 is applied to plate 30. At zero time the button I6 is applied to plate 30 over weld 3|. If the weld is poor the rate of cooling will be relatively slow, as per curve P. If the weld is very good, the rate of cooling will be comparatively rapid, as per curve G. For an average weld, the rate of cooling will be at some intermediate rate, as per curve A.

It is now seen that resistance element l9 will cool to a temperature T1 corresponding to a given instrument deflection in a short time, h, an average time, is, or a longer time, its, depending upon whether the weld be good, average or poor. Thus one way of detecting the nature of the weld will be to note the time required for the instrument to reach a certain deflection.

For timing purposes I have represented an electric clock 33 which is connected to a timing source of supply 34 and for convenience in accurate timing I have shown the clock connection as being made through the metal parts 30 and I6 as a switch when button I6 is contacted with plate 30. When measurements are made as above described. the clock may be provided with a timing dial calibrated from zero time upwards as good," "average and poor.

Instead of holding the detector on the weld until a certain deflection of instrument 29 is obtained, I may maintain the contact for a given short length of time, ta, and read the deflection of Instrument 29. In this case with a good weld the instrument 29 will correspond to a deflection of temperature T3. With a poor weld, the instrument 29 will deflect considerably less in time t3 and will correspond to a temperature T1. In calibrating, welds of known qualities are tested.

It is now seen that I have provided a weld detector which can detectthe quality of the weld when only one surface of the welded structure is accessible. It will, of course, be understood that results will vary with the thickness and kind of material being welded, and the apparatus should be carefully calibrated with suitable curves for different k nds and thickness of material and with various welds, the qualities of which have been carefully determined by other tests, and other conditions which may influence the results. For example, if the room temperature is substantially different than 25 degrees C., say 40 degrees C., it may be assumed that the materials 30 and 3I are at 40 degrees C. and the rate of heat release in a given time from the 200 degree 0. detector II will be less. Adjustment for this may be had by adjustment of the thermostat control I of the heater to hold a correspondingly higher temperature of the mass I I.

The usual calibrating expedients of the circuit of instrument 29 may also be employed. Contact buttons I6 of difierent contact areas and heat storage mass may be used. In order that the device may be used to test different welds in succession without loss of time, the heat storage mass of buttons I6 and II should be small, and small as compared to that of mass II and interve g P r s so that when b t on I5 is remove from the plate 30 .it will quickly return to the temperature of heat storage mass II. Also, there should be no material change in temperature of plate 30 due to the small amount of heat conveyed thereto during a testing operation. Where, as here, the two ends I6 and I! of the head are made identical, it may be advantageous to make alternate tests with alternate ends I6 and Il.

Figs. 5 and 6 represent a tester which is more sensitive in that the heated contact button which is placed in contact with the part to be tested is temporarily moved out of thermal connection with the heat storage mass. In Fig. 5 the heat storage mass Ila corresponds to part II of Fig. 1. I2a represents the electric heater therefor. Illa represents a handle which is connected to part Ila by a ventilated shell part 35. Resistance elements I8 and I9 are the same as in Fig. l but are mounted differently. Resistance element I8 is embedded in the mass Ila while resistance element I9 is embedded in a contact button 36 which is movable in a recess in mass Ila. The button 36 is of metal, is of small heat storage capacity as compared to that of mass Ila, is cone-shaped and is normally held tightly within a coneshaped recess of part Ila by a spring 31 so as to be in good thermal contact with part Ila. The contact button 36 has a small diameter hollow uide stem surrounded by spring 31. The connections to resistance element I9 are through this stem, one being through the material of the stem itself. When the button 36 is held against a plate 30 for test purposes, as in Fig. 6, it is moved upward within the recess to provide a conical-shaped air gap between it and the part Ila so that the good thermal connection between it and the part Ila is broken. Hence, button 36 cools rapidly and little heat is transferred thereto during the testing operation. However, as soon as the test is completed and the button is removed from testing position, spring 31 returns it into good thermal contact with part Ila and it is quickly heated to the temperature of part Ila. A stop strap 38 secured to part Ila limits the upward movement of the button 38 when moved into testing position so that the air gap between it and the part Ila is always the same when in the test condition. The upward movement of button 35 to test position may also serve to close a switch part 39 in the clock circuit.

Fig. 7 illustrates a timing circuit wiring diagram that may be used with the apparatus of Figs. 5 and 6. The'resistance bridge circuit is the same as that previously explained in connection with Fig. 3. The instrument 29, however, hasadjustable contacts 40 and H near the lower and upper ends of its scale and its pointer serves as a movable contact to cooperate with the contacts 40 and H at selected indications.

The clock 33 is driven by a timer motor 42 through a gear train that may be interrupted by lifting the gear 43. The timer pointer 44 is mounted on a shaft 45. A spring 46 which is wound when the pointer 44 is advanced serves to return the pointer against a zero stop 41 when gear 43 is lifted. The gear 43 is lifted to demesh the gear train by an electromagnet 48 when the latter is energized. The electromagnet circuit is through the zero contact 40 and pointer of instrument 29 and through a contactor 49 operated from an extension of shaft 45 of the timer 33'. The contacts 49 serve to open the circuit of electromagnet 48 when the timer pointer 44 is in or near a zero time indicating position at the left of its sca e 50. The upscale contact 4 of instrument 29 is included in the circuit of a relay 9| which when energized opens the circuit 01 the timer motor 42.

The tester of Fig. 5 is used with the system of Fig. 7. With the heater Ila hot and contact 36 in contact therewith, the bridge circuit will be balanced and instrument 29 will read zero and its pointer will engage contact 49. The pointer 44 of the timer will read zero at the left of its scale. Electromagnet 43 will have its circuit open at contact 49. The timer motor circuit will be open at switch 391'" The contact button 36 is now pressed against a part 30 to make a test as in Fig. 6. This closes the timer motor circuit at switch 39 and the timer motor starts and moves its pointer 44 upscale. Contact button 36 rapidly-transfers its heat to part. 30 and unbalances the bridge causing instrument 29 to deflect upscale until it engages contact I I. This energizes relay 6| and stops the timer which thus indicates on its scale the time required for the button 36 to drop in temperature by a given amount. If the flow of heat from button 36 has been rapid, indicating a good weld, the time indication of pointer 44 will be relatively short. Hence, for testing welds the scale 50 of the timer may be, calibrated in terms of good, "average or poor welds reading upward from zero time position, as represented by the scale designations G, "A and "P. As soon as the timer motor is thus stopped, the test is completed and the button 36 may now be withdrawn from the test piece 39. This reopens the timer motor circuit at switch 39 and brings button 36 again into good thermal contact with heat storage mass I la and the button is again quickly heated. The bridge returns to a balanced condition and the pointer of instrument 29 moves from contact ll to contact 40. The timer motor circuit is reclosed at relay 9|, electromagnet l8 is energized, gear 43 is lifted and the timer pointer returns to zero. As soon as pointer 44 returns to zero, electromagnet 48 is again deenergized by opening of its circuit at contact 49. The equipment is now ready for another test.

If the apparatus is used for the testing of sheet metal of a given material for the purpose of determining the thickness, the scale oi. the timer 33 will be calibrated in thickness units with the smallest thickness graduations at the upper end of the time scale. Likewise, if the sheetis ofv a known standard thickness and material, its heat conductivity may be tested.

It is apparent that the apparatus of Figs. 1 and 5 may be used to test spot welds and a variety of other factors. For example, in Fig. 8 I have represented a section of the wall 54 of a boiler containing water 55. Over the interior of section a of the boiler I have represented a thick deposit of scale 56, over section b a medium deposit of scale 51 and over'section o no scale at all. The metal shell 54 is a good heat conductor as also is the water 55. However, the scale is a relatively poor heat conductor and for that reason is detrimental in boilers generally. It is obvious that using my invention I can determine the relative amount of scale deposited on the interior of the boiler shell by applying my test apparatus to the accessible exterior of the boiler.

Thus I have provided apparatus which is applicable to a large variety of test purposes where only one surface of the body or material to be tested is accessible.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. Thermal conductivity testing apparatus comprising a heat storage mass, means for heating said mass and maintaining it at a substantially constant predetermined temperature, a metallic contact member thermally .associated with said heat storage mass having relativelysmall heat storage capacity as compared to that of the heat storage mass and through which heat from said mass is released for testing purposes resulting in an appreciable temperature difference between said heat storage mass and contact member, and means responsive to the temperature difference between said heat storage mass and contact member for obtaining a measurement of the rate oi. heat transfer from said contact member.

2. Thermal conductivity testing apparatus comprising a heat storage mass, means for heating said mass and maintaining it at a substantially constant predetermined comparatively high temperature, a metallic contact member thermally associated with said mass having relatively small heat storage capacity as compared to that .01 said mass and through which heat is released from mass for testing purposes resulting in a substantial cooling of the contact member relative to the mass, and means responsive to the temperature difference between said heat storage mass and contact member for obtaining a measurement of the rate of heat transfer through said contact member.

3. Thermal conductivity testing apparatus comprising a heat storage mass, means for maintaining said mass at a substantially constant higher than average temperature, a metallic contact member thermally associated with said heat storage mass, having small heat storage capacity as compared to said mass, and through which heat is released from said mass for testing purposes, a temperature sensitive electrical.

device subject to the temperature of said contact member, a second temperature sensitive electrical device subject to the temperature of said mass, a circuit including an electrical measuring instrument and said temperature sensitive devices for producing an indication proportional to the difierence in the temperatures of said temperature sensitive devices, and timing means for determining the rate of change of the difference in said temperatures when heat is being released from said contact member.

4. Thermal conductivity testing apparatus comprising a heat storage mass, means for maintaining said mass at a substantially constant, higher than average temperature, a metallic contact member of small heat stdrage capacity as compared to said mass and through which heat is released from said mass for testing purposes, temperature sensitive devices responsive repectively to the temperatures of said mass and contact member, a circuit including an instrumerit and said temperature sensitive devices for measuring the difierence in temperature between said mass and contact member, said contact member being movable to and from good thermal contact with said mass and arranged to so move when withdrawn from and moved to testing position respectively, and means for determining the rate of change in the temperature diflerence between said mass and contact member.

5. Thermal conductivity testing apparatus comprising a heat storage mass, means for maintaining said mass at a predetermined higher than average temperature, a contact member having small heat storage capacity as compared to said mass, said contact member being movable to and from good thermal engagement with said mass and arranged to respectively so move when withdrawn from and moved into engagement with a test piece, a timing device having a scale calibrated in units indicative oi the rate of heat transfer from said contact member, means responsive to the movement of said contact member into engagement with a test piece for starting said timing device and means responsive to a predetermined temperature diflerence between said mass and contact member for stopping said timing device.

7 HANS P. KUEHNI. 

